What Is Surface Roughness?- Unit, Chart, Measurement

Surface roughness is an excellent predictor of mechanical part performance because irregularities on the surface can produce nucleation sites for fractures or corrosion.

In tribology, rough surfaces wear faster and have greater friction coefficients than smooth surfaces. Roughness may be needed in some applications to facilitate adherence to cosmetic finish coatings such as plating, powder coating, or painting.

A suitable surface finish not only makes items appealing but also helps to guarantee that the item works as expected.

To make the best surface finish and manufacturing procedures for your items, you must first learn about surface roughness in manufacturing. Here, we’ll go over all you need to know about surface roughness.

What is Surface Roughness?

Surface roughness is the measurement of the relative smoothness of a surface’s profile, calculated via the microscopic deviations in a surface’s true form. The larger the deviation from its true form, the rougher the surface, whilst the smaller the deviation, the smoother the surface.

Surface roughness, often shortened to roughness, is a component of surface texture. In surface metrology, roughness is typically considered to be the high-frequency, short-wavelength component of a measured surface.

However, in practice, it is often necessary to know both the amplitude and frequency to ensure that a surface is fit for a purpose.

Roughness plays an important role in determining how a real object will interact with its environment. In tribology, rough surfaces usually wear more quickly and have higher friction coefficients than smooth surfaces.

Roughness is often a good predictor of the performance of a mechanical component, since irregularities on the surface may form nucleation sites for cracks or corrosion.

On the other hand, roughness may promote adhesion. Generally speaking, rather than scale-specific descriptors, cross-scale descriptors such as surface fractality provide more meaningful predictions of mechanical interactions at surfaces including contact stiffness and static friction.

Although a high roughness value is often undesirable, it can be difficult and expensive to control in manufacturing.

For example, it is difficult and expensive to control the surface roughness of fused deposition modeling (FDM) manufactured parts. Decreasing the roughness of a surface usually increases its manufacturing cost.

This often results in a trade-off between the manufacturing cost of a component and its performance in application.

Roughness can be measured by manual comparison against a “surface roughness comparator”, but more generally a surface profile measurement is made with a profilometer. These can be of the contact variety (typically a diamond stylus) or optical.

However, controlled roughness can often be desirable. For example, a gloss surface can be too shiny to the eye and too slippery to the finger (a touchpad is a good example) so a controlled roughness is required. This is a case where both amplitude and frequency are very important.

What is Surface Roughness or Roughness?

Surface roughness or roughness is defined as the irregularities which are inherent in the production process (e.g. cutting tool or abrasive grit).

Surface roughness is quantified by the deviations in the direction of the normal vector of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small, the surface is smooth.

What is Waviness?

Waviness is part of the texture on which surface roughness is superimposed. It may result from vibrations, chatter or work deflections and strains in the material. It is also impossible to specify precisely where waviness stops, and the shape becomes part of the general form of the part.

What is Form?

Form is the general shape of the surface, ignoring variations due to roughness and waviness.

Surface Roughness Unit

A common unit of measurement of surface roughness is by measuring the “average roughness”, which is often communicated as “Ra”. Ra is the calculated average between peaks and valleys on a surface.

The lower the Ra value, the less variation between the peaks and troughs on a surface, making the surface smoother. For example, a lego block will have a low Ra value, as will a laptop’s touchpad.

With a higher Ra value, these products would be highly textured and coarser and therefore potentially unsuitable for their desired use.

This contrast of Ra values illustrates the importance of determining the desired surface roughness of a product before the manufacturing process begins. Without such determinations, machining finishes of a product may vary significantly from what was initially intended.

The illustrations below demonstrate the variations between the values of Ra (the numerical average of all the peaks and valleys across the length of the test) and Rz (the average of consecutive highest peaks and lowest valleys).

How to Measure Surface Roughness?

Surface roughness is a calculation of the relative smoothness of a surface’s profile. In this case, there’s the use of a numeric parameter – Ra. Ra surface finish chart shows the arithmetic average of surface heights. The heights have been measured across a surface.

As already mentioned, there are three basic components of a surface. They include the roughness, the waviness, and the lay. Therefore, different factors are affecting the characteristics of surface geometry.

Likewise, there are several measuring systems for surface roughness. Therefore, the systems include:

• Direct measurement methods
• Non-contact methods
• Comparison methods
• In-process methods

The direct measurement methods measure surface roughness using a stylus. Consequently, it involves drawing the stylus perpendicular to the surface. The machinist then uses a registered profile to determine roughness parameters.

Non-contact method methods involve the use of light or sound instead. Optical instruments like white light and confocal replace the stylus. These instruments use different principles for measurement. The physical probes can then be switched with optical sensors or microscopes.

First, the instrument used will send an ultrasonic pulse to the surface. Then, there’ll be altering and reflection of the sound waves back to the device. You can then assess the reflected waves to determine roughness parameters.

On the other hand, comparison techniques employ surface roughness samples. These samples are generated by the equipment or process. Then, the manufacturer uses tactile and visual senses to compare results. The results are compared against the surface of known roughness parameters.

An example of in-process techniques is inductance. This method helps to evaluate surface roughness using magnetic materials. Here, the inductance pickup uses electromagnetic energy. It uses energy to gauge the distance to the surface. Then, the parametric value determined can help find out comparative roughness parameters.

Surface Roughness Chart

There are wide-ranging variations in finishing and edge conditions. Listed are some of the more common manufacturing techniques and their corresponding Ra surface finish values.

Various Methods of Measuring Surface Roughness

There are different methods and equipment involved in measuring surface roughness. The various methods used to fall into three categories. They are:

• Profiling Techniques. Firstly, we have the profiling techniques. This involves the measurement of the surface using a high-resolution probe. In this process, you should think more of a phonograph needle in line with sensitivity. A typical CNC probe may not be as effective.
• Area Techniques. These techniques measure a finite area of the surface. Therefore, the measurement offers a statistical average of peaks and troughs in the surface. Some examples of these techniques include ultrasonic scattering, optical scattering, capacitance probes, and more. It is easier to automate and execute with area techniques.
• Microscopy Techniques. These qualitative techniques rely on measuring contrasts. The results provide relevant information about peaks and valleys on surfaces.

What are the Surface Roughness Measurement Parameters?

In order to predict the behaviors of a component during use or to control the manufacturing process, it is necessary to quantify these surface characteristics by using surface texture parameters.

Surface texture parameters can be separated into three basic types; Amplitude Parameters (vertical characteristics), Spacing Parameters (horizontal characteristics), and Hybrid Parameters (a combination of spacing and amplitude parameters).

Examples of typical surface roughness measurement parameters can be seen below:

• Ra – The universally recognized, and most used, international parameter of roughness. It is the arithmetic mean of the absolute departures of the roughness profile from the mean line.
• Rv – The maximum depth of the profile below the mean line within the sampling length.
• Rp – The maximum height of the profile above the mean line within the sampling length.
• Rt – The maximum peak-to-valley height of the profile in the evaluation length.
• Rz – The maximum peak-to-valley height of the profile within a sampling length.

Why is surface roughness important?

Cleaning

A rough surface has a lot of irregularities and is more sensitive to friction, because if offers resistance to moving objects on the surface. Moreover, dirt could be stuck in the valleys of the metal and that makes it harder to clean.

Smooth surfaces on the contrary, are easy to clean, because of the smooth profile.

That is also one of the reasons why surface roughness plays a great role in industries such as food processing and pharmaceutical industries. The surface is more hygienic and it decreases bacterial adhesion.

Wear

A rough surface is more likely to wear and has a larger amount of friction. The high friction coefficient means that there is more force needed to slide, than for a smooth surface finish.

Anodizing

In order to make the aluminum products’ surface more resistant to wear and corrosion, the electrolytic process, called anodizing, is used.

It increases the thickness of the oxide layer on aluminum parts. However, this anodizing does affect the Ra, since the surface becomes slightly rougher than before the anodizing process.

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